The present invention is generally directed to processes for the preparation of crosslinked thermoplastic or thermoset resin particles. More specifically, the present invention relates to improved polymerization processes which provide crosslinked homopolymer and copolymer resin particle products which have a substantially uniform polymer chain length or primary structure and which polymerization processes proceed with high monomer to polymer conversion. In particular, this invention relates to stable free radical mediated, pseudoliving polymerization processes which yield crosslinked homopolymers and copolymers having low, medium or high crosslink density and improved control of particle size properties, and which processes accomplished in accordance with the present invention provide numerous operational and economic advantages.
This invention also relates to crosslinked thermoplastic resin and preparative processes thereof which provide thermoplastic resin particles that are prepared in situ and can incorporate functional groups that are present in the monomers used. More specifically, the present invention relates to crosslinked polymers, particles thereof, and a process for preparing the polymers and particles directly in the same reaction vessel. This invention also relates to a method for preparing toner compositions which forego energy intensive, conventional mechanical crushing and grinding steps in achieving toner sized particles. The process is particularly useful in the production of thermoplastic polymer resins for use in a wide variety of thermoplastic applications.
The present invention provides, in embodiments, a living or pseudoliving polymerization process that enables the synthesis of crosslinked homopolymer and copolymer resins and particles thereof from a variety of free radical reactive monomers. The process, in embodiments, uses known free radical initiators in combination with nitroxide or non-nitroxide type stable free radical agent compounds, free radical reactive crosslinking compounds containing two or more non adjacent double bonds, and a variety of free radical reactive monomers to afford crosslinked thermoplastic resins or elastomers. In other embodiments, the present invention provides processes for preparing crosslinked thermoplastic or thermoset resin particles having available thereto a broad spectrum of, and operator controllable, level of interpolymer chain crosslinking by conducting a stable free radical mediated polymerization in the presence of one or more suitable crosslinking agent compounds at elevated temperatures, wherein monomer is added substantially uniformly to all of the propagating chains, including main chain or primary polymer structure and side chains effected by the polymerization of the crosslinking compound onto or into the main chain and subsequent polymerization of monomers thereon arising, for example, from integration of the crosslinking compound into another propagating polymer chain.
Of the known polymerization processes a preferred way to crosslink polymers or copolymers is by free radical processes. Conventional free radical polymerization processes that are used to polymerize monomers in general, and functionalized monomers in particular, inherently give broad polydispersity resin products or require that sophisticated processing conditions and materials handling protocols be employed. The use and availability of crosslinked resins having controlled particle size and operator selectable particle composition in industrial applications is limited because known free radical polymerization processes are generally difficult to control and produce, for example, insoluble polymer gels, even if the crosslinking is accomplished in a separate post polymerization step. Consequently such crosslinking and polymerization processes are generally limited in their industrial utility. Furthermore, the extent to which various functional groups can be directly incorporated into the crosslinked polymer is limited because of physical limitations imposed on conventional free radical polymerization reactions, for example, the preparation of sulfonated polystyrene cross linked ion-exchange particles.
It is desirable to have stable free radical mediated polymerization processes which provide crosslinked resin particles which overcome the shortcomings and disadvantages of the aforementioned related free radical polymerization processes.
Crosslinked resins can be constructed with a variety of primary, secondary, and tertiary structures and which structural terms are well known in the art of macromolecular chemistry. The crosslinking associated with these systems provide a number of useful properties, reference for example, commonly owned and assigned U.S. Pat. No. 4,894,309, issued Jan. 16, 1990, to Georges et al., which discloses a free radical suspension polymerization process for obtaining crosslinked styrene butadiene ternary copolymers which comprises the polymerization of styrene monomers, butadiene monomers, and crosslinking components in the presence of a surfactant, a stabilizer, and a first and a second initiator; and wherein the aforementioned components are present in an aqueous phase; heating the resulting aqueous mixture at a temperature of from about 50.degree. C. to about 110.degree. C.; thereafter heating the mixture at a temperature of from about 110.degree. C. to about 150.degree. C.; followed by cooling and isolating the desired product. Mixtures of crosslinked and uncrosslinked resins can be blended in such a manner as to achieve beneficial melt mixing and hot melt rheological properties, such as hot offset temperature and minimum fix temperatures, which properties are of considerable importance and utility to liquid and dry toner developer compositions used in xerographic and related marking technologies, reference for example: U.S. Pat. No. 4,457,998, issued Jul. 3, 1984 to Gruber et al., wherein there is disclosed improved developer compositions afforded by resin particles which are comprised of an uncrosslinked polymer incorporated into a polymer network highly crosslinked in the presence of the uncrosslinked polymer, the uncrosslinked polymer being of a different chemical composition than the crosslinked polymer, thereby resulting in the formation of a sponge-like structure; and U.S. Pat. No. 5,057,392, issued Oct. 15, 1991, to McCabe et al., wherein a crystalline crosslinked and amorphous polyesters, a novolac epoxy resin, and a crosslinking catalyst are admixed, melt blended, and annealed at an intermediate glass transition temperature so that the crosslinked polyester recrystallizes as small highly dispersed particles in a matrix phase comprised of the crosslinked reaction product of the amorphous polyester and the novolac epoxy resin. Other approaches to achieving crosslinked polymeric materials with controlled physical and chemical properties include commonly owned U.S. Pat. Nos. 5,227,460, 5,393,630, 5,401,602, 5,407,772, and 5,414,052.
Sulfonated polystyrene-divinylbenzene (PS/DVB) resins have been commercialized for many years and used as, for example, cation exchange resins in many applications such as in water treatment, in recovery of metals from aqueous solutions, in chromatography, as catalysts, and the like applications. However, their applicability has been restricted due to their low thermal stability in water or water-containing media. Above about 150.degree. C. the aromatic sulfonic acid groups are increasingly hydrolyzed. A number of structurally modified resins have appeared directed toward solving the thermal instability problem and which modified resins reduce the premature loss of sulfonic acid groups when the resins are subjected to elevated temperatures for extended periods of time, reference for example, U.S. Pat. No. 4,728,695, issued Mar. 1, 1988, to Brouwer et al., which discloses crosslinked resin compositions comprising sulfoalkylated-carboxy resins having improved thermal stability properties, that is, when the resin is in the acid form, at elevated temperatures in the presence of water or in aqueous media. Sulfonated polystyrene crosslinked with divinylbenzene and related resins prepared by conventional methodologies, such as free radical polymerization of styrene monomers followed by a post polymerization sulfonation, provides up to only about a 15 mole percent sulfonation of the resin. Hence the ion exchange capacity, that is, the sulfonate groups available for ion exchange processes, of these resins is quite limited.
Practitioners in the art have long sought an inexpensive, efficient and environmentally efficacious processes for producing crosslinked polymers having operator controllable or selectable: crosslink density, particle size range, pore size distribution properties and functional group content, and further, general processes which can be easily modified to selectively afford a wide variety of different crosslinked polymer product types which are tolerant of a wide variety of functional groups with the aforementioned properties, crosslink density, particle size, pore size distribution properties and functional group content.
In the aforementioned commonly owned Georges et al., U.S. Pat. No. 5,322,912, there is disclosed free radical polymerization processes for the preparation of a thermoplastic resin or resins comprising: heating from about 100 to about 160.degree. C. a mixture comprised of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound to form the thermoplastic resin or resins with a high monomer to polymer conversion and a narrow polydispersity. A broad spectrum of free radical reactive monomers are suitable for use in the highly versatile polymerization process. While a variety of homopolymers and copolymers, including block and multiblock copolymers, could be prepared with high conversions and narrow polydispersities, no mention was made or suggested to include a crosslinking agent in the polymerization process to prepare crosslinked polymer resins and particles thereof with the aforementioned desirable resin and particle properties.
The following documents are of interest.
At least two publications, indicate that random crosslinking does not allow for the formation of narrow pore size distributions, reference for example, T. Bein et al., J Phys. Chem., 90, 4851 (1986) for pore sizes on the order of less than about 2 nanometers, and J. L. Garcia et al., J. Magn. Magn. Mater, 363, 140 (1995).
U.S. Pat. No. 4,581,429 to Solomon et al., issued Apr. 8, 1986, discloses a free radical polymerization process which controls the growth of polymer chains to produce short chain or oligomeric homopolymers and copolymers including block and graft copolymers. The process employs an initiator having the formula (in part)=N--O--X, where X is a free radical species capable of polymerizing unsaturated monomers. The molecular weights of the polymer products obtained are generally from about 2,500 to 7,000 having polydispersities generally of about 1.4 to 1.8, at low monomer to polymer conversion. The reactions typically have low conversion rates and use relatively low reaction temperatures of less than about 100.degree. C., and use multiple stages.
European Patent Publication No. 0135280 corresponding to European Patent Application No. EP 84 304,756 is the European Patent Office equivalent of the aforementioned U.S. Pat. No. 4,581,429.
In Polymer Preprints, 35 (1), 778 (1994), Matyjaszewski et al., disclose thermal polymerizations of styrene monomers in the presence of stable radicals and inhibitors, but without a free radical initiator present, such as peroxide or azo compounds.
In the Joumal of the American Chemical Society, 1994, 116, p. 11185-11186, there is disclosed free radical polymerization processes for the preparation of narrow polydispersity polymers, such as polystyrene, and block copolymers, such as poly(styrene-b-acetoxymethyl styrene), using a free radical initiator, such as benzoyl peroxide, a stable free radical compound such as TEMPO, and a monomer, in accordance with the prior teachings of Georges et al., Macromolecules, 1993, p. 26, 2987, which prior teaching is based on the aforementioned commonly assigned U.S. Pat. No. 5,322,912.
In U.S. Pat. No. 5,268,437, to Holy, issued Dec. 7, 1993, there is disclosed a high temperature aqueous processes for the polymerization of monoethylenically unsaturated carboxylic monomer to produce low molecular weight, water-soluble polymer products useful as detergent additives, scale inhibitors, dispersants and crystal growth modifiers. Suitable monomers include acrylic acid, methacrylic acid, maleic acid, maleic anhydride, crotonic acid, and itaconic acid. The reactions are run at temperatures ranging from about 130 to 240.degree. C., preferably from about 140 to about 230.degree. C., with polydispersities less than 2.5. The process can be continuous, semicontinuous, or batch.
In U.S. Pat. No. 4,546,160, to Brand et al., issued Oct. 8, 1985, there is disclosed a process to continuously bulk polymerize acrylic monomers to prepare low molecular weight, uniform polymers employing minor amounts of initiator and, optionally solvents, at short residence times and moderate reaction temperatures to provide high yields of a product with polydispersities less than 3, suitable for high solids applications.
U.S. Patent 3,600,169 to Lawton, issued Aug. 17, 1971, discloses an electrostatic light sensitive reproduction sheet employing a composition comprising in an insulating resinous binder an organic stable free radical and a precursor sensitive to light to be converted to transient free radicals that are reactive with the stable free radical and to change the conductance of the sheet so that an electrostatic image can be formed. Also disclosed is an extensive listing of stable free radical compounds.
Other references cited in an international search report for the aforementioned commonly owned U.S. Pat. No. 5,322,912 are: J. Am. Chem. Soc., 1983, 5706-5708; Macromol., 1987, 1473-1488; Macromol., 1991, 6572-6577; U.S. Pat. No. 4,628,019 to Suematsu et al., issued Aug. 10, 1986; U.S. Pat. No. 3,947,078 to Crystal, issued Aug. 10, 1976; and U.S. Pat. No. 3,965,021 to Clemens et al., issued Jun. 22, 1976. The cited references disclose alternative means, as discussed above, such as anionic, group transfer, and the like, for preparing polymer resins, and in some instances branched or star type polymers, with narrow polydispersity properties, but which alternative means do not provide the convenience and economic advantages of the present invention.
The following references are also of interest: U.S. Pat. Nos. 3,682,875; 3,879,360; 3,954,722; 4,201,848; 4,542,182; 4,581,429; 4,777,230; 5,059,657; 5,173,551; 5,191,008; 5,191,009; 5,194,496; 5,216,096; and 5,247,024.
The aforementioned references are incorporated in their entirety by reference herein.
In free radical polymerization reaction processes of the prior art, various significant problems exist, for example difficulties in predicting or controlling both the polydispersity and modality of the polymers produced. These polymerization processes produce polymers with high weight average molecular weights (M.sub.w) and low number average molecular weights (M.sub.n) resulting in broad polydispersities or broad molecular weight distribution (M.sub.w /M.sub.n) and in some instances low conversion. Further, polymerization processes of the prior art, in particular free radical processes, are prone to generating excessive quantities of heat since the polymerization reaction is exothermic. As the viscosity of the reaction medium increases dissipation of heat becomes more difficult. This is referred to as the Trommsdorff effect as discussed and illustrated in "Principles of Polymerization", G.Odian, 2nd Ed., Wiley-lnterscience, N.Y., 1981, page 272, the disclosure of which is entirely incorporated herein by reference. This is particularly the situation for reactions with high concentrations of soluble monomer, for example greater than 30 to 50 percent by weight soluble monomer, which are conducted in large scale reactors with limited surface area and limited heat dissipation capacity. Moreover, the exothermic nature of free radical polymerization processes is often a limitation that severely restricts the concentration of reactants or the reactor size upon scale up.
Further, gel body formation in conventional free radical polymerization processes may result in a broad molecular weight distributions and/or difficulties encountered during filtering, drying and manipulating the product resin, particularly for highly concentrated reactions.
Other disadvantages associated with the prior art methods for preparing crosslinked polymeric materials is that they typically provide products with highly variable crosslink length or interchain separation, and low functional group content, and highly variable or broad pore size distributions in particles formed thereby, and difficult to control particle size properties, for example.
These and other disadvantages are avoided, or minimized with the crosslinking polymerization and particle forming processes of the present invention.
Thus, there remains a need for crosslinking polymerization processes for the preparation of in situ, high functional group content, easily controlled particle sized polymeric resins by economical and scalable free radical polymerization techniques and which polymers retain many or all of their desirable physical properties, for example, low gel content, processibility, clarity, high gloss durability, and the like, while avoiding the problems of non-uniform crosslink length, gel formation, exotherms, volume limited and multi-stage reaction systems, complex purification, encumbered or compromised performance properties due to undesired residuals, broad polydispersity properties of the polymer resin products, and the like, associated with prior art concurrent free radical polymerization and crosslinking methodologies.
There has been a long felt need for an economical free radical polymerization processes which are suitable for preparing highly functionalized crosslinked type resins in the presence of water or protic solvents.
There also remains a need for polymerization processes which enable the preparation of polymers with high molecular economy and efficiency, and by providing alternative synthetic pathways, such as linear, convergent, and de novo routes, and which compounds and polymerization processes overcome the aforementioned limitations and problems.
There is a need for process methodology to generate stable free radical terminated chain sites which can be used for the purpose of subsequently structurally articulating the terminated site with desirable functional groups and or polymer chain elongation wherein the performance properties of the polymeric product may be enhanced further.
There is also a need for process methodology for crosslinked polymeric materials and particles thereof wherein the crosslink structural elements are generated in situ and de novo.
Processes for the preparation of the aforementioned polymeric materials are enabled, in embodiments of the present invention, by for example, heating a mixture comprising a free radical initiator compound, at least one stable free radical compound, at least one free radical polymerizable monomer, and at least one free radical polymerizable crosslinking compound; and cooling the mixture, wherein the resulting product is crosslinked resin particles comprised of polymerized monomers and crosslinking compounds, wherein the resin has a high functional group content and is prepared in situ in one step.
The polymer resin particulate compositions of the present invention may be formed into a variety of products, for example by known processes such as injection and blow molding processes. Examples of such products include: synthetic catalysts having both chemical and physical properties which enhance the specificity of the catalyst and the reaction rate, such as, the presence of certain functional groups, pore size, surface area, and the like; resins for electrostatographic toner and developer compositions; polymeric materials for applications including, but not limited to, adhesive formulations, surfactants and viscosity modifiers; and monomodal or multimodal resins for use in films and coating technologies.
The polymerization processes and the resultant crosslinked compounds, resins or elastomer particle products of the present invention are useful in many applications, for example, as a variety of specialty applications including toner and liquid immersion development ink resins or ink additives used for electrophotographic imaging processes, and including ink jet ink formulations, or in combination with mixtures of monomodal narrow molecular weight resins or block copolymers which are suitable, for example, in films, electrophotographic marking materials such as toners and toner additives, and aqueous or organic solvent borne coating technologies.
The above and other problems have been unexpectedly solved in embodiments of the present invention wherein there are provided superior results arising from copolymerizing, for example by heating, with an initiator compound, at least one stable free radical compound, at least one free radical polymerizable monomer, and at least one free radical reactive crosslinking agent or monomer to form a crosslinked polymeric product containing crosslinked structural elements which are substantially of the same length, and as illustrated herein.